I am so sorry for you and your lovely boy.
If you're looking for potential timescales, I'm afraid every case is different e.g. a friend of mine had pancreatic cancer, was given 4 months, lived another 3 years. My Boycat had CKD, many cats live pretty long lives with that, he was PTS 9 months after having had a perfect condition MoT & 6 months after 1st symptoms started (again, a young cat only 8 when PTS).
Just keep doing what you are doing for now, not much point in offering him meds or special vet foods if he doesn't like them as you're only prolonging the time he has left by a little but reducing his enjoyment of it. Signs to look out for are the coat losing condition e.g. looking spiky or getting greasy, appetite loss and/or weight loss, calling out for no apparent reason, looking confused (he will be by that stage poor dear), and changes in behaviour e.g. extra cuddly or hiding away lots.
How I handled things with Boycat's CKD was once I could see the end was close I booked him in for PTS for a couple of weeks time, with obviously the proviso that if he deteriorated rapidly I would being him in earlier, and spent that time spoiling him, feeding him his favorites despite them not being particularly good for him like ham (face it, he would be gone before they impacted his health), giving him all the love and cuddles he wanted.
I researched all the options for afterwards e.g. type of cremation, what I wanted to do with the ashes, did I want a posh urn, did I want some ashes in a keepsake like jewellery & wrote it all down so I wouldn't have to think on the day. I booked that day off work plus the following day, and arranged for someone to come with me. Seems morbid, but having everything sorted in advance did help me on the day.
I had no choice really in getting another cat to fill the hole left, as his indoor only litter sister would need a companion (I work out of the home), but maybe have that in the back of your mind. No need to feel guilty as any new cat couldn't replace your boy in your heart & mind, but may help fill the cat shaped hole in your life.
And finally at the end of this long post remember a week to soon is better than a day too late.